U.S. District Court Judge Araceli Martínez-Olguín issued the ruling Wednesday. It covers California, Connecticut, Georgia, Missouri, Nevada, New York, Pennsylvania, Washington and Wisconsin. She put the order on hold until Friday to give the administration time to appeal. If it takes effect, the pause will last until polls close on Election Day.

The case was brought by the California Nurses Association, National Nurses United, the Voto Latino Foundation, San Francisco and Santa Clara County. The judge said the groups had shown that the investigations were harming efforts to assist and mobilize eligible voters. She did not find that union members’ own voting rights had been infringed, noting that most vote by mail.

Court filings say investigators were told to examine naturalized citizens to determine whether they had voted before becoming U.S. citizens. The lists of people flagged for review drew on state and local government information and publicly available data. Martínez-Olguín called the records “voluminous and unreliable,” writing that the country has no federal database of residents’ citizenship status and that rushed reviews increase the chance of mistakes.

During a phase of the investigation that began in late August, agents were directed to spend no more than 12 minutes assessing each person flagged, according to the judge. She said reports that U.S. citizens were mistakenly identified had caused understandable fears of intimidation, discouraging some people from attending campaign events or serving as poll monitors.

The Department of Homeland Security said Thursday that its focus is on “removing aliens who vote and dilute the votes of U.S. citizens.” The department added that it had no plans to question or investigate people it believes are U.S. citizens.

The ruling comes amid broader efforts by the administration to obtain detailed, unredacted voter registration records from states. The Justice Department has sued 30 states that declined to provide the data and, according to the judge, has sought commercial versions of voter lists where states withheld their records. The judge noted that the investigation’s underlying claim is rare, including in the administration’s own investigations.

The probe, called the Unlawful Voter Initiative, became public last month after Senate Minority Leader Chuck Schumer of New York and Sen. Alex Padilla of California released a whistleblower report. Lawyer Beau Tremitiere, who represents the groups challenging the investigation, said the ruling supported their contention that the government relied on bad data and sent agents to people’s homes shortly before an election.